This application is based on French Patent Application No. 01 02 490 filed Feb. 23, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to lasers whose emission wavelength can be varied, which are referred to as tunable lasers. Tunable lasers have been found to be particularly beneficial in optical telecommunication systems using wavelength division multiplexing.
2. Description of the Prior Art
Lasers whose wavelength can be adjusted are already known in the art. Document [1] (see the bibliography at the end of this description), describes a distributed Bragg reflector tunable laser. The laser system includes an amplifier section placed end-to-end with a Bragg section to which an adjustment voltage is applied to tune the laser. Two wavelength adjustment mechanisms have been observed with this laser. One is based on the standard injection of current into the Bragg section and the other is based on the application of a reverse bias voltage to adjust the refractive index electro-optically. The total tuning range of the laser including these two mechanisms about a wavelength of 1.56 xcexcm is approximately 120 angstrom units with 31 modes regularly spaced by 3.5 angstrom units. Of these modes, 25 are obtained by forward biasing the Bragg section (current injection) with a voltage variation of 1.6 volts and six others are obtained by reverse biasing the same section (electro-optical effect) with a voltage variation of 4 volts. With a system of the above kind subject to an electro-optical effect, it has been possible to obtain switching times between two wavelengths of 500 ps, independently of the difference between the switched wavelengths. These switching times are increased to several nanoseconds in the case of current injection.
The tunable lasers most widely used at this time are lasers which can be tuned by injecting a current. The minimum time-delay to obtain tuning is imposed by the lifetime of the carriers in the tuning section, however. According to document [1], short time-delays of 500 picoseconds have been obtained using electro-optical effects. This relatively high speed is achieved at the expense of the tuning range, which according to document [1] is restricted to 25 angstrom units.
A laser that can be tuned using a different principle is described in document [2], which describes a laser in which stepwise tuning is obtained by means of an external cavity delimited by a fiber incorporating a sampled Bragg grating. The laser system shown in FIG. 1 of document [2] includes a laser diode forming a Fabry-Pxc3xa9rot cavity. The laser diode is coupled to an optical fiber including the sampled Bragg grating, which has eight main reflection peaks. The front face of the diode, facing the fiber, includes an anti-reflection layer producing a reflection coefficient of 5xc3x9710xe2x88x923. The resulting laser device operates in a single mode for each of the wavelengths corresponding to a reflection peak of the Bragg grating of the fiber. Monomode operation is possible because of the low reflectivity of the front face, enabling the use of a thin Fabry-Pxc3xa9rot cavity having a free spectrum gap of 103 GHz. The gain condition is met if a residual mode of the Fabry-Pxc3xa9rot cavity has a wavelength coinciding with one of the wavelengths corresponding to a reflection peak of the fiber. Document [2] explains that the device has been used to produce a laser device whose optical emission frequency can be adjusted in steps of 100 GHz, for wavelengths from 1551.09 nm to 1556.66 nm, i.e. a spacing between extreme tuning wavelengths of approximately 6 nm. The wavelength is changed by varying the current injected into the gain medium of the laser diode. A wide range of tuning wavelengths implies injecting high currents and consequently a large variation in emission power. Also, the adjustment obtained by index variation is mainly associated with heating of the structure, so the effect and therefore tunability will be very slow (ms).
Compared to the prior art just described, the invention proposes a tunable laser that can be tuned over a wide range of wavelengths, of the order of 100 nm and more, in a simple manner, by acting on a single control parameter, enabling any optical frequency of the International Telecommunication Union (ITU) chart to be obtained precisely, with a small spacing between consecutive frequencies of 50 GHz or 100 GHz. Thus a tunable laser in accordance with the invention can be tuned over a large number of wavelengths.
The invention aims above all to achieve very short tuning time-delays, for use in optical switching in particular.
The invention also aims to provide a tunable laser of improved longevity that can employ simplified controlled electronics. It further aims to produce narrow-band laser emission of great spectral purity and free of noise.
To be more precise, the invention provides a wavelength-tunable laser including a first resonant cavity containing an optical amplifier medium and a reflector external to the first cavity, delimiting a second cavity, and having reflectivity peaks for an integer number N of optical frequencies, in which laser the first cavity is formed of two opposed reflector members that are not wavelength selective and delimit an amplifying first active section coupled to a phase tuning second active section, each of the two active sections is adapted to be connected to its own electrical supply, the second active section has an effective group index that can be adjusted electro-optically as a function of an electrical voltage applied to it, and the first and second active sections have dimensions such that the difference between the optical frequencies of any two resonant modes of the first cavity is never equal to the difference between the optical frequencies of any two reflectivity peaks of the reflector.
Accordingly, a current injected into the first section defines the gain of the first cavity. Also, because the first cavity is delimited by reflective members that are not wavelength selective, the cavity is a true Fabry-Pxc3xa9rot cavity. The difference between the frequencies of any two adjacent resonant modes is then practically constant, and is imposed essentially by the compositions and dimensions of the members constituting the first cavity. Varying the voltage applied to the second section causes the comb of resonant frequencies of the cavity to slip. The adjustment of these resonant frequencies is the result of an electro-optical effect: the reverse electric field applied through the PIN structure of the phase tuning section imposes a corresponding value of the index of its active layer and therefore of the effective group index of the structure. Depending on the structure chosen, the operative electro-optical effect can be the Franz-Keldysh effect or the Stark quantum confinement effect. Accordingly, the optical length of the first cavity can be modified as a function of the applied voltage, which is what causes the comb of resonant frequencies to slip.
Because the reflector returns toward the first cavity a portion of the light that it emits, and because the first cavity is designed so that the difference between the optical frequencies of any two resonant modes is never equal to the difference between the optical frequencies of any two reflectivity peaks of the reflector, simply adjusting the voltage achieves selective coincidence of only one of the frequencies corresponding to the reflectivity peaks with one of the resonant frequencies of the first cavity, and it is with this single coincident frequency that the laser oscillation of the system occurs.
An external reflector having N reflectivity peaks can be produced by means of a sampled Bragg grating waveguide, as described in document [2] already cited. The grating can consist of a series of samples of constant pitch, regularly spaced along the guide, in which case the coefficients of reflectivity of the peaks as a function of frequency will decrease within an envelope of sine cardinal shape from a center frequency corresponding to the Bragg frequency of the grating. Each sample can also, and preferably, be produced with an index profile along the guide conforming to a sine cardinal function, and this produces reflectivity coefficients of the same value, within a rectangular envelope. This latter embodiment is advantageous in that the laser threshold current condition is the same for all the reflectivity peaks. The current injected into the first section can then remain constant, and adjustments are facilitated. For more details of the design of sampled Bragg gratings, see document [4].
In the particular instance, which is important in practice, where the difference between any two adjacent optical frequencies of reflectivity peaks is constant, the optical frequencies of the reflectivity peaks are advantageously interleaved with consecutive optical frequencies of resonant modes. This feature, still obtained by appropriately dimensioning the first and second active sections, provides adjustment ranges in which the selected frequency (or wavelength) is a monotonous function of the applied voltage. This simplifies voltage control.
In this latter case, the ratio of the difference between two adjacent optical frequencies of two resonant modes to the difference between two adjacent optical frequencies of reflectivity peaks is preferably made equal to N/(Nxe2x88x921). This ensures that the change from one selected frequency to the next is effected by slipping of the comb of resonant modes by a constant pitch. This results in identical selectivity over the whole adjustment range.
In a preferred embodiment, the external reflector is a waveguide including at least one sampled Bragg reflector grating optically coupled to the first cavity.
The sampled Bragg grating waveguide can be a sampled Bragg grating fiber or any other kind of waveguide, in particular a silica or polymer-based planar guide.
Embodiments of the present invention are further described hereinafter with reference to the accompanying drawings.